Be lenient about where to click for REPL

Summary:
This allows you to click anywhere in the "Try it" row instead of having to
click right at the text input line.

Signed-off-by: Ricky Elrod <ricky@elrod.me>

Test Plan: `cabal install` and firefox :)

Reviewers: hvr, carter, johnw, austin

Reviewed By: austin

Subscribers: relrod

Differential Revision: https://phabricator.haskell.org/D53